Estimating γγ absorption for UHE photons with lepton and hadron production

Surprisingly, the contribution to the cosmic opacity to UHE ($E>10^{18} \, \rm eV$) $\gamma$-rays has been systematically computed so far for the $\gamma \gamma \to e^+e^-$ and $\gamma \gamma \to e^+ e^- e^+ e^-$ processes alone. We go a step further by systematically evaluating the additional opacity brought about by other leptons and hadrons. We find that the dominant channels are those leading to the production of $\mu^{\pm}$ and hadrons (mainly $\pi^{\pm}$, $\pi^0$, $K^{\pm}$, $K^0$, $\eta$). For nearly the GZK radius, the photon survival probability becomes smaller by about a factor of two with respect to current estimates.